This invention relates generally to power factor control circuitry for use with a lamp ballast, and more particularly to a power factor control circuit for use with a lamp ballast having an active offset.
Conventional power factor controllers such as model MC34261 and MC33261 available from Motorola Inc. of Phoenix, Ariz. are particularly designed for use in a switch mode power supply of an electronic ballast. The power supply serves to provide a regulated D.C. voltage to power the inverter (commutator) of the ballast.
Generally, the regulated D.C. voltage appears across one or more electrolytic capacitors of the power supply or inverter of the ballast. During no-load or light load conditions of the ballast, that is, when there is substantially no lamp load to draw an appreciable amount of current from the one or more electrolytic capacitors, overcharging of the one or more electrolytic capacitors can occur. Such overcharging makes it more difficult to regulate the D.C. voltage and can damage the capacitor(s) as well as other components (e.g. transistors) of the inverter.
Overcharging of the one or more electrolytic capacitors during no-load conditions (which typically occur during pre-ignition of the ballast or when the load becomes disconnected from the ballast) can be minimized by turning a switch of the switch mode power supply OFF more quickly to minimize the amount of energy transferred to the electrolytic capacitor(s). In other words, the excessive build up of charge across the one or more capacitors during no-load and light load conditions can be substantially eliminated by turning the switch OFF more quickly.
The power factor controller determines when to turn the switch OFF based, in part, on a current sensed signal representative of the current flow through the switch. This signal is fed into a current sensed input of the controller. In order to increase the speed at which the switch turns OFF thereby minimizing potential overcharge of the electrolytic capacitor(s), a passive (i.e. constant) offset is added to the current sensed signal.
The enhanced sensitivity of the power factor controller to no-load and lightly loaded conditions through provision of a constant offset results in a higher switching frequency of the switch-mode power supply. The amount of energy transferred to the electrolytic capacitor(s) is reduced thereby avoiding an excessive build up of charge across the latter. The constant offset, however, can also reduce the power factor of the ballast and, in particular, leads to a higher than desired total harmonic distortion (THD) level in current drawn by the ballast.
Accordingly, it is desirable to provide a power factor controller which is sensitive to no-load and light load conditions of the lamp ballast to prevent overcharging of the electrolytic capacitor(s) while minimizing the THD level in current drawn by the ballast.